1. Field of Invention
This non-provisional application claims the benefit of U.S. Provisional Application No. 60/038,293 filed Feb. 20, 1997, entitled "Focusing Device", by Touru IWANE. The present invention relates to a focus adjustment apparatus which accomplishes automatic adjustment of the focus state in a camera or other optical equipment, and more particularly relates to a focus adjustment camera which easily and accurately predicts the complex motion of the image plane position.
2. Description of Related Art
Conventionally, focus adjustment apparatuses are mounted on optical equipment such as cameras in order to perform automatic focus adjustment of the photographing optical system.
In these focus adjustment apparatuses, the distance between the object image plane and the imaging plane (hereafter, this distance will be referred to as the "defocus amount") is computed using a commonly known phase difference detection method or the like.
The focus adjustment apparatus accomplishes focus adjustment by scrolling the photographing optical system forward and backward in accordance with this defocus amount.
Normally, in this kind of focus adjustment, two operation modes, namely the "single AF mode" and the "continuous AF mode" are used.
That is to say, in the single autofocus (AF) mode, focus adjustment is instantly halted (focus lock) at the point in time when an in-focus determination is made during the focus adjustment period. On the camera side, the photographing operation is started after waiting for this focus lock state. This kind of single AF mode is primarily used in focus-priority photographing.
In contrast, the continuous AF mode continues focus adjustment regardless of an in-focus determination. On the camera side, the photographing operation is executed in conjunction with the operation of a release button by the photographer. This kind of continuous AF mode is primarily used in photography giving priority to shutter opportunities.
However, when the object is moving, the object moves an extra amount during an interval beginning when the release button is completely depressed and ending when the photographing operation is performed (the so-called release time lag), and thus a defocus occurs.
Hence, as one type of the above-described focus adjustment apparatuses, a device has been known which extrapolates the trend of the focus detection information prior to release in the continuous AF mode or the single AF mode and predicts the image plane motion during the release time lag. By performing focus adjustment on the basis of this prediction, it is possible to correct the defocus caused by the release time lag.
Furthermore, the defocus amounts are detected one-by-one via an electric charge accumulation process in the light-receiving element array and a computation process. Consequently, the defocus amounts are data that are detected discretely and time delayed.
This time delay in the data results in useless time (dead time) in the control operation. Consequently, inadequate focus adjustment can easily occur in the subsequent operation of focus adjustment.
Hence, as one type of the above-described focus adjustment apparatuses, a device has been known wherein the image plane position of the current point in time is gradually predicted by extrapolating the trend of the past focus detection information in the continuous AF mode.
By performing focus adjustment continuously in conjunction with this predicted value, inadequate focus adjustment performed in the control operation of focus adjustment is compensated for, making it possible to obtain improved focus adjustment performance.
As described above, accurately predicting the image plane position is extremely important in attempting to improve the performance of focus adjustment.
A method is known for linearly predicting the image plane position (the so-called first-order hold and second-order hold of the discrete value) on the basis of the past data of the image plane position. See, e.g., the disclosure of Japanese Laid-Open Patent Publication 62-263728.
However, in this kind of conventional example, the image plane position is predicted on the basis of the assumption that the image plane speed is a constant or the image plane acceleration is a constant.
Consequently, a problem arises in actual cases, wherein when the image plane speed and image plane acceleration change, it is impossible to predict the image plane position accurately.
In particular, even in the relatively simple motion model where the object moves uniformly on the object space side, the image plane position in a conjugate position moves in a complex manner. Consequently, even for objects having relatively simple motion, it is extremely difficult to accurately predict the motion of the image plane position.
Furthermore, the image plane acceleration is computed from two differences in the image plane position. Consequently, it is easy for phase delays with the image plane position and high-range noise components to adversely affect the prediction of motion of the image plane position. Accordingly, in the conventional prediction method, the problem arises that the accuracy of the predicted value is low, acutely reflecting the effects of detection scattering of the focus detection information.